Some Q's about using propane, butane and isobutane as refrigerants

[Big SturL]

Today I went and bought some bottles of propane/butane mix (70%/30%), which I'm going to use for brazing, and I'm also planning on using it as a refrigerant.

As far as I've understood, using a low-pressure gas, will help in condensing a higher-pressure gas, and therefore allowing me to use a weaker compressor and a smaller condensor, than if I only used the higher-pressure gas.

Now, the butane has a boiling point of only -0.5C, whilst, the propane boils at -42C. Do you guys think using butane as an auxiliary condenser (proper term?) for propane will help much?

I've also found this mix of propane, butane and isobutane (or, since this is VPCC: R290/R600/R600a). I don't know the percentages here, but I at least know the isobutane boils at -10C. Would this be a better mix?

[Brettbeck]

Any of those will condense fairly easy. I assume you will be using the gas in a single-stage unit? If so, then I don't think there is any need for an auxiliary condenser, or to mix the gases really. R600 isn't really a good option for refrigeration because it doesn't boil low enough. R600a is used in domestic fridges quite a lot, but r290 would be a better option on its own in a single-stage.

[Big SturL]

It will be in an SS yes. The use of butane was to help condense the propane EVEN easier, so that one could use a very small compressor and condenser, and still achieve good temps.

HOW greatly the butane will affect this, I don't know, that's where the leet's come into play.

[[XC] gomeler]

Interesting gas but you'd be better off with just a charge of r290, those butanes will just hamper your performance. You could try liquid charging the system, would have to figure out what state the seperate gasses would be in, densitites, and hope there is a way to easily seperate them within the cylinder.

[Big SturL]

Hmm... Will the butane greatly affect the boiling temperature?

A lot of the reason for me asking these questions is due to what I read in the Autocascade drawings thread. Some of you might remember the user "mytekcontrols", a fellow that seemed to have a touch with auto-c's. He believed that R290 and R600 would be a good blend, as the R600 would help in condensing the R290.

[Brettbeck]

R600a is sometimes used in autocascades because of the higher discharge temperature and pressures. Because the R600(a) can condense at a much higher temperature, it cools down the condenser which helps to condense the first refrigerant (such as r290 for example). You need this more in autocascades so that the first phase seperator will work more effectively.

[[XC] gomeler]

Hmmm I thought the R600 was used as an auxillary cooler. You basically use a large R600 charge to max out the heat-dissipation abilities of your condenser and then evaporate off the R600 in an HX with the rest of the Autocascade charge, dropping it down below room temperature. Can't imagine R600 condensing before the condenser like you say and evaporating in the 200psi atmosphere of the highside with decent temps, but I'm no professional.

[SexyMF]

I've also found this mix of propane, butane and isobutane

I think I know what you have found. Camping stove bottles.
I really suggest using Propane from the 16oz bottles (same size as MAPP). And making the adapter from a cheap nozzle and access valve.

[Clemmaster]

It will be in an SS yes. The use of butane was to help condense the propane EVEN easier, so that one could use a very small compressor and condenser, and still achieve good temps.

HOW greatly the butane will affect this, I don't know, that's where the leet's come into play.

Keep in mind that the lower the pressures are doesn't mean the system will work properly . That's the main probleme people who make 3-stages cascade with ethylen-ethylen have : ethylen can condense in the second stage, and the HX2-temp is under -80°, a this temperature the ethylen doesn't need >5bar to condense and so you just get a little mass condensed, and you have to use big compressors with high displacement and flowrate to carry any load. You'll get the same problem here, you should ever be >8bar on discharge pressure, compressors are designed that for, propan condense very easily and such gases as R12-R600a are used with passiv condensers as the ambiant temperature is around 45° and then the system work properly

[Brettbeck]

Hmmm I thought the R600 was used as an auxillary cooler. You basically use a large R600 charge to max out the heat-dissipation abilities of your condenser and then evaporate off the R600 in an HX with the rest of the Autocascade charge, dropping it down below room temperature. Can't imagine R600 condensing before the condenser like you say and evaporating in the 200psi atmosphere of the highside with decent temps, but I'm no professional.

R600 could be used as an auxiliary HX, but R600a would be a much better choice. YOu don't really need one in a single stage though, it wouldn't benefit the performance much if at all.

If you have say an autocascade with r600a, r290, r744 or whatever, the discharge pressure will be quite high and so will the discharge temperature, so the condenser will have to remove more heat for the r290 to condense. If you look at the PT chart for r600a;




Basicly it will condense even if not much heat is being removed (e.g 15bar @ 85c), and once that condenses it will lower the condenser temperature, making it easier to condense the first stage refrigerant (such as r290), and once it enters the phase seperator you can be sure all the r290 has condensed. And of course the better seperation the more effective your 1st HX will be .

I hope this isn't straying too far away from the subject .

[Big SturL]

Well, I have plans of constructing an auto-c as well. Would the propane be usable as an aux-condenser here, or would isobutane, with its lower boiling point, be a far better choice?

[Brettbeck]

Do you mean the auxiliary HX on the single-stage or autocascade? For single-stage I wouldnt bother with an auxiliary HX.

For these systems, propane is more a first stage refrigerant. Isobutane is more of an auxiliary HX gas. It just helps to lower the temperature inside the high side. You don't have to mix, but if anything use propane on its own for the single-stage, and mix a small amount of isobutane in there too. It may help if the condenser isn't that good.

[Big SturL]

No, I was thinking of a propane/co2 auto-c, where I would use butane as the aux-condenser for the propane.

[LukeXE]

No, I was thinking of a propane/co2 auto-c, where I would use butane as the aux-condenser for the propane.

No need for aux condenser for propane.

I`m using always only propane-butane mixes in autocascades and never got problems with that mix.

You can try to use aux condenser for co2 or just make slhx before separator and that`s it.